1. Field of the Invention
The present invention relates to a deformable mirror which is capable of deforming continuously a shape of a reflecting surface, and particularly to a small-size deformable mirror which is manufactured by applying a semiconductor-manufacturing technology.
2. Description of the Related Art
A structure in which a deformable mirror is used by sealing inside thereof upon reducing pressure (upon depressurizing) has hitherto been proposed (for example, refer to “The membrane mirror as an adaptive optical element”, Journal of the Optical Society of America, Volume 67, No. 3, March 1977). FIG. 23A shows a perspective view of a conventional deformable mirror 10, and FIG. 23B shows a cross-sectional view of the conventional deformable mirror 10. Moreover, FIG. 24 shows another cross-sectional view of the deformable mirror 10.
As shown in FIG. 24, in the deformable mirror 10, a ground (GND) is formed on a membrane (thin film) 11 on which the reflecting surface is formed. A plurality of drive electrodes 13 is formed at intervals at positions facing the GND. Moreover, a voltage is applied between the drive electrodes 13 and the GND. Accordingly, the membrane 11, in other words the reflecting surface, is deformed by an electrostatic force which is generated.
Moreover, a glass lid 12 for sealing upon vacuumizing is provided facing a surface on which the reflecting surface is formed. An electroconductive thin film 14 is formed on a surface of the glass lid 12 facing the reflecting surface. A voltage is also applied to the electroconductive thin film 14. Accordingly, the reflecting surface can be deformed to a concavo-convex by combining the electrostatic force in which the drive electrode 13 is used. Accordingly, it is possible to change a direction of reflection of a beam L.
In a case of an electrostatic drive in which the electrostatic force is used, a gap between the GND and the drive electrode 13 is narrow. Therefore, at the time of driving the membrane 11, air in the gap becomes a damper (air damping). As a result of this, response characteristics of the membrane 11 are affected adversely. Therefore, in the deformable mirror 10, the pressure is reduced to approximately 2.0 Torr, and an effect of the air damping is suppressed.
However, when the deformable mirror is used in a vacuum environment, a pressure in the sealed-inside changes in course of time due to a problem in sealing technology. With the change in the pressure, the response characteristics of the deformable mirror are changed. Consequently, a problem that a desired response cannot be achieved arises.